The present invention relates to a method for the integral bonding of two semiconductor silicon wafers or, more particularly, to a so-called SOI (silicon-on-insulator) wafer and a method for the preparation thereof.
It is known that, when two mirror-polished semiconductor silicon wafers or a two silicon wafers of which at least one is provided with an oxidized surface film of silicon formed on the surface are brought into contact at the mirror-polished surfaces under clean conditions, the wafers adhere one to the other even without using any adhesive or other bonding means. This phenomenon is referred to as joining hereinafter. This condition of joining, however, is not a state of complete bonding so that the wafers can be firmly bonded together only after a heat treatment of the wafers under junction. This state of firm bonding is referred to as "integral bonding" hereinafter. An SOI wafer as the subject matter of the present invention is obtained by integrally bonding two semi-conductor silicon wafers, of which at least one has an oxidized surface, with the oxidized surface film or films interposed between the wafers.
Such an SOI wafer can be freely subjected to subsequent treatments such as a high-temperature heat treatment and various types of chemical treatments because no foreign substances such as an adhesive and the like need not be interposed between the component wafers and has an advantage that formation of a pn-junction and embedding of a dielectric material can be accomplished efficiently and conveniently. Accordingly, SOI wafers are highlighted in recent years as a practically applicable item along with the rapid progress in the technology of thin film formation relative to the flatness and cleanness of the surface.
The progress in the technology of semiconductor devices in recent years to accomplish higher and higher degrees of integration and higher and higher operating velocities has led to a trend in the SOI wafers of which the thickness of the active layer is decreasing. For example, an SOI wafer of which the thickness of the active layer for the formation of a semiconductor device is as small as around 0.1 .mu.m is now required. An SOI wafer having such an extremely thin active layer can never be prepared by a conventional mechanical working method such as grinding and polishing. Suitable thickness-reducing methods for finishing include, for example, the etch-stop method by using a high-concentration layer of dopant reported by K. Imai in Japanese Journal of Applied Physics, volume 30 (1991), page 1154, the dry etching method disclosed in Japanese Patent Kokai 5-33539 and so on.
The above mentioned etch-stop method with a high-concentration layer of dopant is described by making reference to FIGS. 3A to 3F of the accompanying drawing, each of which illustrates one of the steps (A) to (F), respectively, in the method by a schematic cross sectional view of the wafers. In step (A), as is illustrated in FIG. 3A, an ordinary semiconductor silicon wafer 1 having a thickness of 600 to 1000 .mu.m as a substrate, which is referred to as the base wafer hereinafter, is provided with an oxidized surface film 1A having a thickness of 1 .mu.m or smaller on the mirror-polished flat surface. On the other hand, a second silicon wafer 2 having a thickness of 600 to 800 .mu.m, which is referred to as a bond wafer hereinafter, is subjected to a doping treatment with a boron dopant on the mirror-polished surface by the thermal diffusion method or ion implantation method to form a high-concentration doped layer 2A to serve as the active layer for the formation of a semiconductor device, of which the thickness is about 0.5 .mu.m and the concentration of the dopant is around 10.sup.20 atoms per cm.sup.3.
In step (B), in the next place, the two wafers 1 and 2 after the above described pretreatment are brought into contact each with the other at the respective mirror-polished surfaces, as is illustrated in FIG. 3B, in a clean atmosphere at room temperature to be joined together with the oxidized film 1A and the dopant-diffused layer 2A in direct contact followed by a heat treatment at a temperature in the range from 700 to 1000.degree. C. in an atmosphere of oxygen for about 1 hour to effect integral bonding of the wafers 1 and 2. In this heat treatment, diffusion of the dopant proceeds through the bond wafer 2 so that the thickness of the doped layer in a high dopant concentration is increased while the concentration thereof is somewhat decreased.
In step (C) of the process, the integrally bonded body of the wafers 1 and 2, i.e. a precursor of an SOI wafer, is subjected to mechanical grinding or polishing to thin down the bond wafer 2 until the thickness thereof is decreased into a thin layer having a thickness of 5 to 10 .mu.m, as is illustrated in FIG. 3C.
In step (D), in the next place, the integrally bonded body of wafers 1 and 2 after the mechanical grinding or polishing treatment on the bond wafer 2 in step (C) is subjected to an etching treatment with an aqueous etching mixture prepared by blending 3400 ml of ethylenediamine, 600 g of pyrocatechol and 1600 ml of water at a temperature of 100 to 110.degree. C. so that etching of the bond wafer layer rapidly proceeds and to remove the portion with a low dopant concentration so that the dopant concentration at the etching front is succeedingly increased to gradually decrease the rate of etching until etching is stopped at a certain level of the dopant concentration. By this etch-stop method, the bond wafer 2 is removed by etching leaving only a thin layer 2A of a relatively high dopant concentration having a thickness of about 0.5 .mu.m. as is illustrated in FIG. 3D.
In step (E), in the next place, the surface of the bond wafer 2 or the dopant-diffused layer 2A is subjected to a thermal oxidation treatment to form an oxidized film 2B on the surface. The dopant is taken into this oxidized surface film 2B under growing so that the concentration thereof in the dopant-diffused layer 2A is decreased to one tenth to one twentieth provided that the conditions for the growth are adequately controlled. Thus, a thin layer of silicon 2A having a thickness of 0.1 to 0.2 .mu.m, referred to as an SOI layer hereinafter, is formed between the above mentioned oxidized surface film 1A for integral bonding and the oxidized film 2B grown on the surface, as is illustrated in FIG. 3E.
Finally, in step (F), this oxidized surface layer 2B is removed by etching with diluted hydrofluoric acid so as to give an SOI wafer having an extremely thin SOI layer 2A exposed on the surface, as is illustrated in FIG. 3F.
In connection with the method for the preparation of such an SOI wafer having an extremely thin SOI layer 2A, however, a trouble is sometimes encountered during the procedure of the mechanical grinding or polishing for obtaining a very thin SOI layer 2A as mentioned above or during the procedure for the preparation of a semiconductor device by using the SOI wafer that exfoliation or separation between layers takes place due to incomplete bonding. When two wafers 1 and 2 are joined together to prepare an integrally bonded wafer through the heat treatment, for example, separation of the two wafers takes place in the course of grinding and polishing to decrease the thickness of the bond wafer 2 to 5 to 10 .mu.m as in usual cases.
In the case of an integrally bonded wafer obtained by bonding a bond wafer having a high-concentration doped layer on the mirror-polished surface and a base wafer after oxidation of the mirror-polished surface, in particular, troubles are sometimes caused that separation takes place between the bonded surfaces due to weakness of the bonding strength.
The inventors have conducted studies for finding the mechanism of these troubles to arrive at a discovery that, when two mirror-polished wafers without a high-concentration layer of dopant on the surface are to be bonded together, the surface roughness of each of the bonded surfaces as mirror-polished does not exceed, for example, about 10 bits as estimated in terms of the haze level based on the scattering intensity of a laser beam. A satisfactory integrally bonded SOI wafer can be obtained when such mirror-polished wafers with the above haze level are joined together after thermal oxidation of one or both of these wafers followed by a bonding heat treatment.
It has been discovered that the surface of a wafer having a layer doped with boron or antimony in a high concentration, however, is necessarily roughened in the course of thermal diffusion or ion implantation of the dopant so that no satisfactory integrally bonded wafer can never be obtained from such a wafer having a roughened surface due to occurrence of voids in a large number. The bonding strength in an SOI wafer having such voids is low so that separation of the layers frequently takes place in the course of the polishing process for thickness reduction of the SOI layer or in the course of the preparation of a semiconductor device after thickness reduction of the SOI layer.